The invention disclosed herein generally relates to voltage regulation, and more particularly relates to switched mode power converter, e.g., as implemented by an N-level Buck architecture.
Multilevel converter structures are known in power converter applications. For example, flying-capacitor converter topology, hereafter noted FCC, was introduced in 1992 for inverter applications. See, e.g., T. A Meynard, H. F. (1992) “Multilevel Conversion: High Voltage Choppers and Voltage-Source Inverters,” Power Electronics Specialists Conference (pp. 397-403); Toledo: IEEE. FCC topology provides a lower output voltage ripple, reduced harmonic distortion, and lower electromagnetic concerns. See, e.g., A. Shukla, A. G. (June 2007); “Capacitor Voltage Balancing Schemes in Flying Capacitor Multilevel Inverters,” Proc. IEEE Power Electron., vol. 6, no. 1, pp. 2367-2372.
An exemplary FCC power converter, e.g., shown in FIG. 1, includes four power switches (two high side switches and two low side switches, all of which are serially connected between VBAT and ground), an inductor L, and a Flying capacitor CMID. The inductor is connected between an output node and a midpoint node located between the high and low side switches. The capacitor is connected in parallel with two of the power switches. The FCC power converter can be configured in four operational configurations or states, e.g., those shown in Table 1 below, when the voltage across the flying capacitor VCMID equals ½VBAT. Both states S1 and S2 generate the level ½VBAT on the LX node located at the midpoint between the power switches, e.g., between the two high side switches and the two low side switches. States S1 and S2 are hereafter called redundant states, and thus this FCC converter is commonly referred to as a 3-level FCC converter.
TABLE 1Multilevel power converter ConfigurationsSTATEM0M1M2M3V(LX)S0ONONOFFOFFVBATS1OFFONOFFON½ VBATS2ONOFFONOFF½ VBATS3OFFOFFONON0The functionality of the FCC converter relies on correct balancing of the flying capacitor. For a 3-level FCC converter, the flying capacitance is maintained at a voltage of ½VBAT. Several techniques exist to address this requirement.
“Self-Balancing of the Clamping-Capacitor-Voltages in the Multilevel Capacitor-Clamping-Inverter under Sub-Harmonic PWM Modulation” by X. Yuan, H. S. (March 2001, IEEE Trans. Power Electron., vol. 16, no. 2, pp. 256-263) explores “natural-balancing” techniques for FCC. Active regulation techniques, such as modifying the duty-cycle or redundant state selection (RSS), have also been described in literature. For example, “A Soft-Switching High-Voltage Active Power Filter with Flying Capacitors for Urban Maglev System Applications by B. M. Song, J. S. (2001, Conf. Rec. IEEE IAS Annu. Meeting, vol. 3, pp. 1461-1468) maintains the flying capacitor voltage by adjusting the duty cycle according to the error between the measured voltage and a reference. “Active Capacitor Voltage Balancing in Single-Phase Flying Capacitor Multilevel Power Converters” by M. Khazraei, H. S. (February 2012, IEEE Trans. Ind. Electron., vol. 59) presents a regulation technique for an N-level FCC using RSS. In this regulation scheme, the presence of redundant states for generating some voltage levels (VBAT/2 in the 3-level case) is leveraged for the regulation of the flying capacitor. When several states are available to generate a given output level, some of the states will charge the flying capacitance while other states will discharge the flying capacitor. The charging/discharging property of a given state is dependent on the current polarity in the inductor. Thus, based on the voltage level of the flying capacitor, the direction of the current in the coil, and the voltage level to be generated at the output, it is possible to select the best operational state for maintaining the desired flying capacitor voltage level.
Conventional power converters were mainly developed for power inverters, and thus for applications running with discrete switching elements operating under hundreds of volts and at switching frequencies of 1-10 kHz. Further, conventional power converter solutions use discrete Hall Effect sensors, which tend to be large and require a large footprint on a circuit board, to implement the requisite current sensing in the coil. When using power converters in mobile communication devices, however, the control system requires higher speeds (e.g., switching frequencies in the range of 50-200 MHz), lower cost, and smaller footprints.